COMMON THINGS COLOUR. 



must be understood before it is possible that the causes of colour 

 can be rightly comprehended. 



2. In relation to the production of light, bodies are considered 

 as luminous and non-luminous. 



3. Luminous bodies, or luminaries, are those which are original 

 sources of light) such, for example, as the sun, the flame of a lamp 

 or candle, metal rendered red-hot, the electric spark, lightning, 

 and so forth. 



Luminaries are necessarily always visible when present, pro- 

 vided the light they emit be strong enough to excite the eye. 



4. Non-luminous bodies are those which themselves produce no 

 light, but which may be rendered temporarily luminous when 

 placed in the presence of luminous bodies. These cease, however, 

 to be luminous, and therefore visible, the moment the luminary 

 from which they borrow their light is removed. Thus the sun, 

 placed in the midst of the planets, satellites, and comets, renders 

 these bodies luminous and visible ; but when any of them is 

 removed from the solar influence by the interposition of any object 

 not pervious by light, they cease to be visible, as is manifest in 

 the case of lunar eclipses, when the globe of the earth is inter- 

 posed between the sun and moon, and the latter object is therefore 

 deprived of light. A candle or lamp placed in the room renders the 

 walls, furniture, and surrounding objects temporarily luminous, and 

 therefore visible ; but if the candle be screened by any object not 

 pervious to light, those parts of the room from which light is inter- 

 cepted would become invisible, did they not receive some light 

 from the other parts of the room still illuminated. If, however, 

 the candle or lamp be completely covered, all the objects in the 

 room become invisible. 



5. In relation to the propagation of light, bodies are considered 

 as transparent and opaque. Bodies through which light passes 

 freely are called transparent, because the eye placed behind them 

 will see such light through them. Bodies, on the contrary, 

 which do not admit light to pass through them, are called opaque ; 

 and such bodies consequently render a luminary invisible if inter- 

 posed between it and the eye. 



Transparency and opacity exist in various bodies in different 

 degrees. Glass, air, and water are examples of very transparent 

 bodies. The metals, stone, earth, wood, &c. are examples of 

 opaque bodies. 



Correctly speaking, no body is perfectly transparent or perfectly 

 opaque. 



6. There is no substance, however transparent, which does not 

 intercept some portion of light, however small. The light is thus 

 intercepted in two ways ; first, when the light falls upon the 



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